Tracing the Face and Age of the Placental Mammal Ancestor

A small, furry-tailed, insect-eating creature was the earliest ancestor of placental mammals—a widely diverse group of animals ranging from bats to humans—according to a new study by a team of international scientists, including a core group of Museum researchers.

In findings published today by the journal Science, the researchers analyzed the world’s largest dataset of genetic and physical traits to find that placental mammals diversified into present-day lineages much later than is commonly thought: after the extinction event 65 million years ago that eliminated non-avian dinosaurs.

There are two major types of data for building evolutionary trees of life: phenomic data, which includes anatomical and behavioral observations gleaned from physical specimens, and genomic data from DNA analyses. The new study combines genomic and phenomic data in a simultaneous analysis for a more complete picture.

An artist’s rendering of the hypothetical placental ancestor, a small insect-eating animal.

Carl Buell

“Despite the considerable contributions of DNA sequence data to the study of species relationships, phenomic data have a major role in the direct reconstruction of trees,” said author Michael Novacek, senior vice president, provost for science, and a curator of paleontology at the American Museum of Natural History. "Such data include features preserved in fossils where DNA recovery may be impossible. The mammalian record is notably enriched with well-preserved fossils, and we don’t want to build trees without using the direct evidence that these fossils contribute."

The tree of life produced in the study shows that placental mammals arose rapidly after the Cretaceous-Paleogene (KPg) extinction, with the original ancestor speciating about 36 million years later than predicted by a commonly held theory. This finding, and the visualization of the placental ancestor, was made with the help of a cloud-based and publicly accessible database called MorphoBank, supported primarily by the National Science Foundation. With this tool, the researchers recorded traits—characteristics like the presence or absence of wings, type of hair cover, and brain structures—for 86 placental mammal species, including 40 species known only from fossils.